1. Field of the Invention
This invention relates to an apparatus for providing thermal insulation around a nuclear reactor pressure vessel while at the same time permitting the reactor vessel to be surrounded passively with cooling water in the event of a severe accident in which the core material relocates.
2. Background Information
The pressure vessel of a nuclear reactor that houses the nuclear fuel must he provided with biological shielding to protect personnel and equipment in the surrounding area from the high neutron flux and radiation produced by the fission reactions within the core of the reactor. Typically, in commercial nuclear power plants, this biological shielding is provided by a massive concrete structure within a containment that defines a cavity or well in which the reactor vessel is suspended by supports under its inlet nozzles. In order to protect the concrete walls of the reactor well from high temperatures and for thermal efficiency of the process, the reactor vessel is provided with thermal insulation. Typically, the thermal insulation is applied directly to the outer surface of the reactor vessel. This requires removal of the thermal insulation for periodic external vessel inspection of the vessel welds, U.S. Pat. No. 5.268,944 suggests an arrangement in which an octagonal configuration of insulating panels is placed around the upright cylindrical reactor vessel to provide a space in which remotely operated inspection equipment can externally monitor the welds without the removal of the insulation.
In addition to the thermal insulation, it is common to circulate cooling air over the walls of the reactor cavity. This cooling air flows out of the top of the reactor cavity and then laterally through the vessel supports.
Another consideration for design of nuclear power plants is neutron streaming, which is the flow of neutrons upward out of the reactor cavity. Typically, neutron absorbing material has been provided around the reactor vessel above the inlet and outlet nozzles. However, the farther from the core that the neutron stream is intercepted, the greater the area of protection that is required.
The assignee of this invention has developed a design for a nuclear power plant that is provided with passive protection systems. The passive protection systems do not require human intervention to respond to abnormal conditions in the operation of the reactor. The safety systems provide for a passive response to a severe accident which is postulated as a meltdown or relocation of the reactor core. It has been determined that the integrity of the reactor vessel can be maintained if the vessel can be immersed in cooling water which is free to vaporize and cam off the heat as steam. One of the passive systems currently available utilizes the large volume of refueling water maintained within the containment for flooding the vessel cavity. For a severe accident in which the core of the reactor melts, it is essential that the large volume of cooling water be applied directly to the vessel.
There is a need, therefore, for an apparatus that can respond to a severe accident and passively immerse the reactor vessel directly in cooling water which is free to vaporize and disperse from the reactor cavity.
There is a further need to continue to provide air cooling for the reactor cavity walls, but without cooling the reactor vessel itself.
There is also a need for protection against neutron streaming to locations outside of the reactor cavity.
There is also a need to simultaneously accommodate all of the above needs economically and reliably without the need for human intervention.
The foregoing needs were satisfied for an AP600 nuclear plant designed by the assignee of this invention by the vessel insulation system described in U.S. Pat. No. 5,699,394. However, it was also determined that the described insulation system would not be sufficient to adequately cool an AP1000 pressure vessel (also designed by the assignee of this invention) in the unlikely event of a nuclear core relocation. Therefore, there is a need to simultaneously accommodate all of the above needs economically and reliably without the need for human intervention for an AP1000 nuclear power generating facility.